1. Field of the Invention
This invention relates generally to furnace controls, and more particularly to an automatic vent and fuel valve control for a fluid fuel-fired furnace.
2. Description of the Prior Art
Conventional, domestic gas-fired furnaces comprise a combustion chamber communicating with a draft hood which, in turn, communicates with a vent or stack. A heat exchanger is typically located above the combustion chamber, and a gas line having a solenoid-operated valve therein extends into the combustion chamber and terminates in a nozzle or burner. In hot air furnaces, a blower is provided for circulating air through the heat exchanger.
In conventional control systems for furnaces of the hot air type, a thermostat sensing a predetermined lower temperature in the space being heated closes its contacts to energize the gas valve. A fan and limit switch senses the temperature in the heat exchanger and when the temperature therein has risen to a lower predetermined level, the fan and limit switch energizes the blower. When the temperature in the space being heated rises to a predetermined higher value, the thermostat opens at the contacts thereby de-energizing the gas valve; however, the blower continues to operate for a period of time to extract heat from the heat exchanger and it is then de-energized by the fan and limit switch. The fan and limit switch will also deenergize the gas valve if the temperature of the heat exchanger reaches a predetermined higher limit, the gas valve remaining closed until the blower has cooled the heat exchanger down to the lower limit.
In the past, no damper was provided in the furnace vent or stack and it will readily be seen that a substantial amount of heat was lost through the stack after the burner was shut-down. Automatically operated vent dampers have been provided to closeoff the vent pipe or stack after the burner has been shut-down thus retaining some of the heat in the heat exchanger which normally would escape through the vent and flue as lost heat. Such prior automatic vent dampers have been of the normally-open type, i.e., biased to the open position by a weight, and have been closed by a motor or solenoid in response to shuttingdown of the burner. Various cam and microswitch arrangements have been employed for detecting the damper position; however, such mechanical arrangements are subject to mechanical wear and temperature extremes.
Present automatic damper control systems known to the present applicant do not provide for opening the damper in response to sensing the presence of hydrocarbon-containing gas in the vent or draft hood, such as would be caused by a downdraft in the flue which tends to blow carbon monoxide back into the dwelling, or the sensing of raw gas in the event that the burner fails to light or if the flame is accidently extinguished. It is therefore desirable to provide an automatic vent damper and valve control system which will sense the presence of hydrocarbon-containing gas, close the gas valve and open the damper in response thereto.
It is further desirable that such a control system close the damper after a predetermined time delay following shuttingdown the burner in order to permit the escape of excess hydrocarbon through the flue and also to accommodate certain types of delayed-closing gas valves.